There is dramatically more heat capacity in the ocean than in the atmosphere. Many power plants reject heat directly to the ocean, and clearly they do produce a local hotspot, which is much argued over by ecologists.

There can be some valid local ecological concerns. If it emits into a spot which is particular important to some species it can become a problem. It can also create ecological activities, where some species congregate around the outlet because they want the hotter water. This isn't necessary good, but again, it's a local occurrence.

Globally, using ocean water for cooling is almost always ecologically better, simply because it's more efficient than alternatives that are either constrained in terms of the usable volume flow of water, or must use a less efficient medium for heat transfer. Eventually, all heat we produce will be radiated into space, but if an activity indirectly produces greenhouse gases, then it warms by a very large multiplier compared to its direct heat production.